1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image (including characters and the like) on a recording medium (plain paper, coat paper, an OHP (Over Head Projector) sheet, glossy paper, a glossy film, a color-image-only paper, a cut sheet, such as a postcard or an envelope) after which the image is fixed and discharged. The present invention relates to an image forming apparatus, such as a printer, a facsimile machine, a copying machine or the like, for mainly using an electrophotographic technology to form a color image on a recording medium, fix the color image and discharge the recording medium. More particularly the present invention relates to a fixing technology for use in the image forming apparatus capable of forming either matte or glossy images on two sides of a recording medium. Further, the present invention relates to a paper discharge passage of the image forming apparatus.
The present application is based on Japanese Patent Application Nos. Hei. 9-203850, Hei. 9-203851, Hei. 9-282810, Hei. 9-282811, Hei. 9-282812, Hei. 9-282813, Hei. 9-282814 which are incorporated herein by reference.
2. Description of the Related Art
In general, image forming apparatuses have widely been known which incorporate a fixing unit for conveying a recording medium while heating and applying pressure to the recording medium. The apparatus is arranged such that a non-fixed image formed by a developer, such as toner, is formed on the surface of the recording medium after which the recording medium is allowed to pass through a fixing unit having a pair of rollers so that an image is fixed to the recording medium.
Devices for use with an image forming apparatus of the above-mentioned type have been known, for example, as in Japanese Patent Publication No. 5-48917 and Japanese Patent Publication No. 6-68646. These devices attempted to improve the fixing characteristic by selectively (as necessary) allowing a recording medium to pass through a fixing unit two times.
One of the two types of the image forming apparatuses which has been disclosed in Japanese Patent Publication No. 6-68646 has a double-side mode for forming images on two sides of a recording medium and a single-side mode for forming an image on either side of the recording medium. When a glossy image is formed on an OHP film in the single-side mode, the OHP film is allowed to pass through the fixing unit two times. Thus, a glossy image can be obtained.
Specifically, the apparatus disclosed in Japanese Patent Publication No. 6-68646 has a structure in which a plurality of recording mediums, on which images are formed on one side, are temporarily accumulated in an intermediate tray so that images can be formed on both sides of the recording medium, in the double-side mode. Each recording medium is again supplied from the intermediate tray to the image forming portion so as to form an image on the other side. Thus, images are formed on both sides of each recording medium. However, when a glossy image is formed on an OHP film, a plurality of the OHP films are temporarily accumulated in the intermediate tray. Then, each OHP film is again fed to the fixing unit from the intermediate tray. But, when a plurality of OHP films are stacked and accumulated in the intermediate tray, the OHP films easily adhere to one another. Because the OHP films easily adhere to one another, there arises a clogging problem in the feeding operation, or undesirable stacking easily takes place.
Further, when a separating agent is applied to the fixing roller, in Japanese Publication No. 6-68646, the separating agent adheres to the surface of the recording medium. Since the recording medium in the form of a resin sheet, such as an OHP sheet, has a poor oil absorbing characteristic, the OHP sheets furthermore easily adhere to one another when the OHP sheets are stacked on the intermediate tray. Thus, problems easily arise in the feeding operation, or stacking easily takes place. Also, since a space for the paper feeding tray is required, there arises another problem in that the size of the apparatus cannot be reduced.
When a color image is formed, toner images in a plurality of colors (for example, yellow, magenta, cyan and black) superimposed on the recording medium must be melted and fixed. Since a great heat capacity is required, a known fixing unit is structured such that a recording medium is allowed to pass through the fixing unit at relatively low speed. See for example, Japanese Patent Publication No. 6-40235.
An image forming apparatus capable of forming images on two sides of paper sheets, a two-side copying machine as disclosed in Japanese Patent Publication No. 61-11865, for example, has been known.
As shown in FIG. 9(a), a typical double-side copying machine has a structure that an original document 1 is irradiated with a lamp 2. Reflected light is imaged on a photosensitive drum 3 so that an electrostatic latent image is formed on the photosensitive drum 3. The electrostatic latent image is visualized into a powder image or the like by a developing unit 4 so as to be transferred to copying paper 7 fed from paper feeders 5 and 6. The powder image is then fixed to the copying paper 7 by a fixing unit 8. The copying paper 7 is allowed to pass through a passage 9, and is then moved to a right-side/reverse-side converting unit 10. If a single-side final copy is desired, the copying paper 7 is allowed to pass through a paper-discharge passage 11 and a deflector 12. The copying paper 7 is then discharged to the upper surface of a paper-discharge tray 13 disposed on the body of the apparatus.
If a double-side final copy is desired, the copying paper 7 having a powder image formed by the fixing unit 8 is allowed to pass through the passage 9, and is then moved to the right-side/reverse-side converting unit 10. After the leading end of the copying paper 7 has temporarily been introduced into the paper-discharge passage 11, the copying paper 7 is moved inversely such that the copying paper 7 is allowed to pass through an inverse passage 14. Thus, the copying paper 7 is stacked and accommodated in an accommodating portion 15. The operation for stacking copying paper in the accommodating portion 15 continues until images are copied on one side of a predetermined number of paper sheets.
After the operation for copying one side of all of the copying paper sheets has been completed, the copying paper 7 stacked in the accommodating portion 15 is allowed to pass through a re-feeding passage 17 by a paper-supply roller 16 so as to again be moved to the photosensitive drum 3. A powder image is then formed on the other side, after which the copying paper 7 passes through the fixing unit 8 as well as the right-side/reverse-side converting unit 10 and is moved to the paper-discharge passage 11.
FIG. 9(b) is a diagram showing an example of the right-side/reverse-side converting unit described above. The right-side/reverse-side converting unit has a drive roller 18 and conveying rollers 19, 20 disposed at the inlet and outlet portions thereof. A drive roller 21 and a movable roller 22 are disposed in the rear of the right-side/reverse-side converting unit. A deflecting plate 23 is disposed between the drive roller 18 and the drive roller 21. The deflecting plate 23 is rotatively supported by a shaft 23a such that the deflecting plate 23 is always urged clockwise by a spring (not shown).
The operation of the right-side/reverse-side converting unit will now be described.
(i) When the copying paper 7 moved from the fixing unit 8 through the passage 9 is moved to the paper-discharge passage 11, all of the rollers are rotated in a direction (forward direction) indicated by an arrow shown in the drawing. Thus, the copying paper 7 discharged from the fixing unit 8 is moved to the paper-discharge passage 11 by the drive roller 18, the conveying roller 19, the drive roller 21 and the movable roller 22.
(ii) When passage of the trailing end 7a2 of the copying paper 7a moved in the forward direction through the leading end of the deflecting plate 23 has been detected by the detecting portion 24, the drive roller 21 and the movable roller 22 disposed in the rear of the detecting portion 24 are reversely rotated. The trailing end 7a2 of the copying paper 7a is guided by the deflecting plate 23 so as to be introduced into a position between the drive roller 18 and the conveying roller 20.
(iii) When the trailing end 7a2 of the copying paper 7a is detected by detecting portion 25, thereby indicating that the copying paper 7 has reached a position between the drive roller 18 and the conveying roller 20, the movable roller 22 is moved upwards to separate it from drive roller 21.
Once a space is formed between the drive roller 21 and the movable roller 22, the next copying paper 7b is conveyed to the paper-discharge passage 11 by the drive roller 18 and the conveying roller 19. At this time, a leading end 7b1 of the next copying paper 7b is guided by the upper surface of the preceding copying paper 7a.
(iv) When the leading end 7a1 of the preceding copying paper 7a has been allowed to pass through the position between the drive roller 21 and the movable roller 22, the movable roller 22 is moved downwards. Moreover, the drive roller 21 is rotated forward so that the next copying paper 7b is conveyed until the trailing end 7b2 of the copying paper 7b is allowed to pass through the detecting portion 24.
The above-mentioned operation is then repeated as necessary.
After the image has been formed on either one or both sides of a paper sheet, it must be discharged from the apparatus. An image forming apparatus therefore not only incorporates an image forming portion, but also incorporates a paper supply portion, a paper discharge passage and a receiving portion. The paper supply portion may include a paper feeding cassette or a paper supply tray, for supplying paper sheets to the image forming portion. The paper discharge passage discharges the paper sheet on which an image has been formed by the image forming portion. The receiving portion, such as a paper discharge tray, receives the paper sheet discharged from the paper discharge passage.
In a typical apparatus, a paper-sheet conveying passage (including the paper discharge passage) is formed from the paper supply portion to the receiving portion through the image forming portion. The paper sheet is sometimes clogged in the paper-sheet conveying passage.
If paper sheets are still conveyed after one paper sheet has been clogged, the clogging is exacerbated. The clog may be exacerbated to the point where there is a risk of breaking the apparatus. Therefore, an apparatus of the foregoing type is usually provided with a detection means for detecting clogging of a paper sheet. If the detection means detects clogging of a paper sheet, the operation of the apparatus is interrupted. Moreover, a message (alarm sound and/or display on a display portion of an operation panel) indicates occurrence of clogging to a user. Then, the apparatus awaits removal of the clogged paper sheet, whereupon the apparatus again becomes operational.
Some apparatuses are capable of forming images at high speed. These image forming apparatuses mainly use electrophotographic technology and sometimes encounter continued conveyance of paper sheets in the conveying passage even when a paper sheet is clogged therein. In this case, the user is undersirably caused to remove paper sheets which are not clogged. Removal of the sheets which are not clogged is a demanding task.
To overcome the problem of requiring a user to remove the sheets which are not clogged, a copying machine has been suggested in Examined Japanese Patent Publication No. 6-1394. There, a drive mechanism of a conveying system is started to discharge the paper sheets, which are not clogged, to a tray. The discharge of sheets occurs after clogging has taken place and a copy start button is again depressed.
&lt;Problem 1&gt;
In a conventional image forming apparatus, a recording medium allowed to pass through a fixing unit is undesirably curled.
The conventional image forming apparatuses disclosed in Japanese Patent Publication No. 5-48917 and Japanese Patent Publication No. 6-68646 have a structure in which the recording medium is not turned inside out when the recording medium, having an image formed on one side thereof, is allowed to pass through the fixing unit two times. Therefore, curls of the recording medium are accumulated and enlarged, causing the recording medium to easily clog in the conveying passage. As noted above, a clogged conveying passage is undesirable.
In recent years, users have required an apparatus for fixing a matted image (delustered image) or a glossy image (a bright image) upon demand. A matted image is obtained by preventing complete melting of the toner image. A matted image can thus be obtained by fixing the matted image with a relatively small quantity of heat. A glossy image is obtained by completely melting the toner image. Therefore, a glossy image can be obtained by fixing the toner image with a relatively large quantity of heat.
The conventional technologies also encounter problems in meeting user requirements for both matted and glossy images as follows.
A first type of apparatus is structured such that the recording medium selectively passes through the fixing unit two times. Examples of the type of apparatus are disclosed in Japanese Patent Publication No. 5-48917 and Japanese Patent Publication No. 6-68646). This type of apparatus attempts to form a matted image by passing the recording medium, having a non-fixed image formed thereon, through the fixing unit only one time when a matted image is required. When a glossy image is required, this type of apparatus passes the recording medium through the fixing unit two times. However, in both cases, image (either matted or glossy) which is formed on the recording medium is brought into contact with only the same roller.
When a matted image having an excellent image quality is required, it is preferable that the surface of the image be somewhat rough by not completely melting the toner particles on the surface of the image. To cause a somewhat rough state in the toner particles, it is preferable that the surface layer of the roller, which is brought into contact with the surface of the image, have a relatively low hardness.
When a glossy image having an excellent image quality is required, it is preferable that the toner on the surface of the image be completely melted and smoothed. To smooth the surface of the toner it is preferable that the hardness of the surface layer of the roller, which is brought into contact with the surface of the image, have a relatively high hardness.
Thus it can be seen that the above-mentioned conventional technologies having a structure in which the image (either matted or glossy) formed on the recording medium is brought into contact with the same roller, cannot easily and selectively form a matted image and a glossy image. If the hardness of the surface layer of the roller is made to be relatively low, a glossy image having a satisfactory image quality cannot be obtained even though an excellent matted image can be obtained. Similarly, if the hardness of the surface layer of the roller is made to be relatively high, a matted image having an excellent image quality cannot be obtained even though a satisfactory glossy image can be obtained.
The above-mentioned problems are also experienced with the conventional technology disclosed in Japanese Patent Publication No. 6-40235. Thus, the conventional technologies based on one roller having a set hardness, cannot selectively form a matted image or a glossy image having an excellent image quality.
&lt;Problem 2&gt;
Apparatuses arranged such that the recording medium is allowed to selectively pass through the fixing unit two times are disclosed in Japanese Patent Publication No. 5-48917 and Japanese Patent Publication No. 6-68646. These apparatuses have a structure in which the recording medium is allowed to pass through the fixing unit one or two times under the same fixing condition (fixing pressure, fixing temperature and fixing speed). Therefore, a matted image or a glossy image cannot selectively be obtained.
If a fixing condition is set such that a matted image can be obtained in one pass, a glossy image cannot easily be obtained after the passing operation is performed two times under the same fixing condition. The reason is that heat is radiated from the recording medium and the toner image during the period between the first pass and the second pass. Although a glossy image can be obtained by performing the passing operation two or more times, there is risk that an excessively large number of times may be required to do so.
If the fixing condition is set such that a glossy image can be obtained by performing the passing operation, for example, two times, the quantity of heat is too much to obtain a matted image after the passing operation has been performed one time under the same fixing condition.
As can be understood from the foregoing description, the conventional technologies, based on the number of times a sheet is fed, cannot easily and selectively obtain a matted image or a glossy image.
Another conventional technology, disclosed in Japanese Patent Publication No. 6-40235, attempts to form a matted image by allowing a recording medium having a non-fixed image formed thereon to pass through a fixing unit at relatively high speed. When a glossy image is required, the apparatus attempts to form such glossy image by passing the recording medium through the fixing unit at relatively low speed. However, this type of conventional apparatus is arranged such that the recording medium is allowed to pass through the fixing unit only one time and thus cannot easily obtain a glossy image having a high image quality although a matted image can be obtained.
Where a glossy image is obtained by allowing a recording medium to pass through a fixing unit at low speed, a large quantity of heat is supplied. If a large quantity of heat is supplied over a long period of time, so-called offset easily takes place wherein toner adheres to the fixing unit (for example, a fixing roller). Thus, the quality of the image easily deteriorates.
Therefore, the above-mentioned conventional technology, based on sheet feeding speed, cannot easily and selectively obtain a matted image or a glossy image having an excellent image quality. When a glossy image is required, another problem arises in that the conveying speed of the recording medium, is reduced undesirably.
&lt;Problem 3&gt;
The double-side copying machine disclosed in Japanese Patent Publication No. 61-11865 as shown in FIG. 9 (b) is structured such that when the next copying paper 7b is introduced simultaneously with the discharge of the preceding copying paper 7a from the right-side/reverse-side converting unit 10, the leading end 7b1 of the next copying paper 7b is guided by the upper surface of the preceding copying paper 7a. Therefore, the reverse side (the side on which no image has been formed) of the next copying paper 7b and the upper surface (the side on which an image has been formed) of the preceding copying paper 7a are brought into slidable contact with each other.
This structure gives rise to a problem in that both the reverse side of the next copying paper 7b and the upper surface of the preceding copying paper 7a are contaminated (as a result of which both of the sides of copying paper sheets following the copying paper 7b are contaminated).
If the copying paper is plain paper which has been electrostatically charged or thin paper, or if the copying paper is paper, such as a postcard or an envelope, which can strongly be curled, the leading end 7b1 of the next copying paper 7b cannot appropriately be guided by the upper surface of the preceding copying paper 7a. As a result, there arises a problem in that clogging of paper easily takes place.
The copying machine disclosed in Japanese Patent Laid-Open No. 6-1394 is structured to simply discharge paper sheets, that follow the paper sheet which has caused clogging, to the tray. Similarly to the conventional and usual image forming apparatuses, the copying machine in Japanese Patent Laid-Open No. 6-1394 has a structure in which the operation for forming images is temporarily interrupted if a paper sheet is clogged.